1. Field of the Invention
This invention relates to an electrode for applying an electric voltage to an optical channel waveguide of an optical waveguide element in which the optical channel waveguide is formed by proton exchange, and a method of forming the electrode.
2. Description of the Related Art
There have been provided various optical waveguide elements having an optical channel waveguide formed on a substrate. As a method of forming the optical channel waveguide, there has been known a proton exchange process.
In the proton exchange process, metal film is first formed on a surface of a substrate, an opening is formed in the metal film by etching and proton exchange is carried out on the surface of the substrate using the metal film as a mask.
Generally an electric voltage is applied to the optical channel waveguide through electrodes disposed near or just above the optical channel waveguide.
A conventional method of forming the electrodes for applying an electric voltage to the optical channel waveguide will be described with reference to FIGS. 3A to 3H, hereinbelow.
Metal film 2 such as of Cr is first formed on a substrate 1 as shown in FIG. 3A.
A resist layer 3 is formed on the metal film 2 in a predetermined pattern by photolithography as shown in FIG. 3B.
Then the metal film 2 is etched to form openings 4 in a predetermined pattern in the metal film 2 using the resist layer 3 as a mask, and the resist layer 3 is removed as shown in FIG. 3C.
Thereafter proton exchange is carried out using the metal film 2 with the openings 4 as a mask, thereby forming optical channel waveguides 5 on the surface of the substrate 1 as show in FIG. 3D.
The metal film 2 is then removed by etching as shown in FIG. 3E and the substrate 1 is annealed as required.
Thereafter a conductive film 7 such as of aluminum is formed over the surface of the substrate 1 as shown in FIG. 3F.
A resist layer 8 is formed over the conductive film 7 with portions opposed to the optical channel waveguides 5 exposed by photolithography as shown in FIG. 3G.
Then the conductive film 7 is removed at the portions opposed to the optical channel waveguides 5 by etching using the resist 8 as a mask as shown in FIG. 3H.
When the resist 8 is thereafter removed, the conductive films 7 are left on opposite sides of each optical channel waveguide 5. The conductive films 7 on opposite sides of each optical channel waveguide 5 can be used as electrodes for applying an electric voltage to the optical channel waveguide 5.
However this method is disadvantageous in the following point. That is, when the resist mask 8 is formed over the conductive film 7 with the portions opposed to the optical channel waveguides 5 exposed, the edge of the resist mask 8 circumscribing the optical channel waveguide 5 cannot be precisely aligned with the edge of the optical channel waveguide 5 due to fluctuation in skill of the operator and/or in precision of the exposure device. Accordingly, the edges of the electrodes (conductive film) hang over the optical channel waveguide or are positioned away from the edge of the optical channel waveguide 5 as shown in FIG. 4 in an enlarged scale, which results in fluctuation in performance of the optical waveguide element or deterioration in yield. In FIG. 4, L denotes the alignment error.
In Japanese Unexamined Patent Publication No. 7(1995)-146457, there is disclosed a method of forming the electrodes for a optical waveguide element which can overcome such a problem. In the method, the metal film which is used as a mask for setting the pattern of the optical waveguide upon proton exchange is left there and used as the electrodes. That is, metal film is formed on a surface of a substrate, openings of predetermined shapes are formed in the metal film, proton exchange is carried out on the surface of the substrate with the metal film used as a mask, thereby forming optical channel waveguides, and the metal film is removed with at least a part of the edges of the openings left there. The metal film fractions are used as the electrodes.
In the method, the metal film is generally of Ta or Cr suitable for proton exchange. However such metals are high in specific resistance and accordingly optical waveguide elements provided with such electrodes are hard to operate at high speed.
When the aforesaid metal film is formed of Au which is low in specific resistance in order to overcome such a problem, then the following problem arises.
When the Au film is processed to form therein openings of predetermined shapes, wet etching is generally employed since Au is hard to process by dry etching. When the Au film is processed by wet etching, the edges of the openings formed become large in roughness and the dimensional accuracy of the optical waveguide deteriorates, which results in fluctuation in performance of the optical waveguide element or deterioration in yield.
On the other hand, when the Au film is processed by liftoff in order to smoothen the edges of the openings formed, the dimensional accuracy the optical waveguide can be higher than when the Au film is processed by wet etching. However since Au particles are apt to adhere to each other, Au particles released into the solution during liftoff adhere to the Au film fractions which are left there as the electrodes, which can cause short circuit or the like. Further in this case, dust is apt to adhere to the surface of the Au film fractions during proton exchange and/or the subsequent processes and bonding of pad electrodes to the Au film fractions deteriorates. The similar problems arises when precious metal such as Pt is employed in place of Au.